1. Technical Field
The invention relates to regeneration of diesel exhaust particulate traps and more particularly, to a system and method for introducing heat, fuel and oxidizer to the particulate trap to induce combustion of particulate buildup.
2. Description of the Problem
Diesel engines generate unburned hydrocarbons (HC), carbon monoxide (CO), carbon dioxide (CO2), nitrogen oxides (NOx), and particulates, and can pass molecular oxygen (O2) with the exhaust product. The particulate matter is principally solid particles of carbon and metal compounds with adsorbed hydrocarbons, sulfates and aqueous species. Among the adsorbed species are aldehydes (e.g. formaldehyde) and polycyclic aromatic hydrocarbons. Particulates, and carbon monoxide, are principally byproducts of incomplete combustion. Combustion in the engine can be modified to minimize particulate generation, however, doing so with contemporary technology leads to increases NOx emission.
Nitrogen oxides are also considered undesirable. NO is produced in large quantities at the high combustion temperatures which promote complete combustion and avoid particulate generation. NO2 is formed principally by the post oxidation of NO in the exhaust. NO2 production can be reduced by retarding engine timing and exhaust gas recirculation, both of which again contribute to particulate generation. Low emissions of nitrogen oxides also favor generation of CO and leave unburned hydrocarbons.
A currently favored approach to lowering diesel emissions is to accept increased particulate levels in the exhaust stream from the engine in order to reach the NOx targets, and to introduce a particulate trap to the exhaust stack to then remove the particulates and achieve the desired emission levels. Such traps can be constructed of metal or ceramics, and include a filter capable of collecting particulates from the exhaust stream. The trap must be able to withstand high temperatures, which are introduced to the traps periodically to oxidize particulate deposits which form in the traps. It has not proven easy to maintain traps at a sufficient temperature to burn the carbon deposits, especially where the traps are located a substantial distance from the engine compartment and downstream from exhaust energy recovery devices, such as power turbines for turbocharging systems. The problem is further complicated when the diesel is operating under a partial load. Secondary measures such as electrically powered heaters, which consume a great deal of power, have been built into particulate traps to address this problem. Particulate oxidation is further promoted by the injection of fuel into the exhaust stream which burns on contact with the heaters. This method of course increases fuel usage.
As part of tighter motor vehicle emission standards, commercial operators will be required to avoid long term idling of diesel engines. Extended idling has been a common practice for drivers who park their rigs for mandatory break periods and desire to have power for heating, cooling and entertainment while on break.